1. Field of the Invention
The present invention relates to a combined service and parking brake apparatus capable of being used as a vehicular brake apparatus, capable of generating braking force by means of brake fluid pressure not only when the apparatus is used as a service brake but also when the apparatus is used as a parking brake, and configured such that, when the apparatus is used as a parking brake, after generation of braking force, the apparatus is mechanically locked to maintain braking force.
2. Description of the Related Art
A combined service and parking brake apparatus of this type is disclosed in, for example, Japanese Kohyo (PCT) Patent Publication No. 2000-504811. The combined service and parking brake apparatus shown in FIG. 4 of the publication includes a piston disposed in a cylinder in such a manner as to be nonrotatable about a cylinder axis and movable along the cylinder axis to thereby form a hydraulic chamber within the cylinder; a brake lining to be moved under pressure by the piston and engaged with a rotary member to be braked, when the piston is moved under the pressure of brake fluid fed into the hydraulic chamber, thereby effecting a braking operation on the rotary member; and adjusting nut attached, within the hydraulic chamber, to the cylinder in such a manner as to be rotatable about the cylinder axis and movable along the cylinder axis; a friction clutch provided between the adjusting nut and the cylinder, allowing rotation of the adjusting nut in an unlocked condition where the adjusting nut is as its home position, and disabling rotation of the adjusting nut in a locked condition where the adjusting nut has moved along the cylinder axis by a predetermined distance from its home position; an adjuster including an external-thread portion of an adjusting spindle provided integrally with the piston and an internal-thread portion provided integrally with the adjusting nut and meshed with the external-thread portion, capable of automatically adjusting operation timing of the friction clutch through automatic adjustment of the clearance between the piston and the adjusting nut in accordance with the amount of wear of the brake lining; and an electromagnetic actuator capable of holding the friction clutch in an engaged condition while the brake lining, having moved under pressure applied by the piston, is clamping the rotary member to be braked.
In the combined service and parking brake apparatus of the above-mentioned publication, by means of deactivating (de-energizing) the electromagnetic actuator, the friction clutch can be held in a disengaged condition, thereby allowing rotation of the adjusting nut and movement, along the cylinder axis, of the piston connected to the adjusting nut via the adjuster. Accordingly, at this time, by means of feed of brake fluid into or drainage of brake fluid from the hydraulic chamber, the piston can be advanced or retreated along the cylinder axis; i.e., the apparatus can be operated as a service brake.
By means of activating (energizing) the electromagnetic actuator, the friction clutch can be held in an engaged condition, thereby preventing rotation of the adjusting nut and thus preventing movement, along the cylinder axis, of the piston connected to the adjusting nut via the adjuster. Accordingly, the combined service and parking brake apparatus can be operated as a parking brake as follows. In a condition where brake fluid is fed under pressure into the hydraulic chamber, and the piston is advanced along the cylinder axis (a braking state where the brake lining is moved under pressure applied by the piston and engaged with the rotary member to be braked), the electromagnetic actuator is activated, thereby brining the friction clutch into an engaged condition and thus preventing rotation of the adjusting nut and movement, along the cylinder axis, of the piston connected to the adjusting nut via the adjuster. Accordingly, even when brake fluid is drained from the hydraulic chamber, the braking state can be maintained; i.e., the brake apparatus can operate as a parking brake.
In operation of the combined service and parking brake apparatus of the above-mentioned publication as a parking brake, while brake fluid is fed under pressure into the hydraulic chamber, the electromagnetic actuator is activated, thereby bringing the friction clutch into an engaged condition. In this case, a meshed portion between the external-thread portion of the adjusting spindle and the internal-thread portion of the adjusting nut is in a stretched condition. In subsequent release of brake fluid pressure (in halt of feed of brake fluid into the hydraulic chamber), reaction force of the brake lining causes the piston to be pressed back by a thread-to-thread clearance present in the meshed portion between the external-thread portion of the adjusting spindle and the internal-thread portion of the adjusting nut. This unavoidably causes a drop in parking brake force.